Frequency converters are being increasingly used for wind energy installations. Modern wind energy installations normally have a variable rotation speed, as a result of which the generator produces alternating current at a different frequency. Frequency conversion is necessary for feeding into a fixed-frequency supply grid (normally 50 Hz). Frequency converters are used for this purpose. In this case, it is preferable to use converters which consist of a generator-side inverter which is electrically connected to the rotor of the generator, a DC voltage intermediate circuit and a grid-side inverter which is electrically connected to the grid. Energy installations have normally been operated such that they are disconnected from the grid in the event of grid faults, in particular short circuits. With the increasingly widespread use of wind energy installations and the increase in the installed wind power, the requirement is, however, no longer simply for real current to be fed into the electrical grid, but it is desirable for the wind energy installations to be operated to support the grid voltage. This grid voltage therefore comprises feeding in real power even under unfavourable conditions, such as discrepancies in voltage or discrepancies of the grid voltage from the nominal value. In the case of reduced voltage or reduced grid frequency, a supporting operation is desirable. In the case of the doubly-fed asynchronous generators which are frequently used for relatively high-power wind energy installations, one problem that can arise is that the converter is designed for only about one third of the electrical power of the wind energy installation and can no longer achieve the additionally required currents for supporting the grid.
It is known from DE 10 2007 028 582 A1 for the generator-side inverter in the case of a generator with a full converter, via which all of the electrical power is fed into the electrical grid, to be connected directly to the grid, to be precise when the generator is no longer generating power and the grid-side inverter can therefore be used to feed additional reactive power into the grid. This concept has the disadvantage that the desired support can be provided only when the generator is not outputting any power.